Corrosion inhibition method suitable for use in potable water

ABSTRACT

Effective corrosion inhibitors having minimal organic content, particularly suitable for use in municipal drinking water systems, are described. The corrosion inhibiting additive is a stannous salt of a non-carbon acid.

[0001] This application is a continuation-in-part of allowed U.S.application Ser. No. 09/360,768, filed Jul. 26, 1999, which claimspriority to U.S. provisional application Ser. No. 60/114,551, filed Dec.31, 1998, now abandoned, both of which are hereby incorporated byreference in their entirety.

FIELD OF THE INVENTION

[0002] The present invention relates to methods for inhibiting corrosionof corrodible metals present in contact with water in distributive watersystems, particularly those associated with distribution of drinkingwater.

BACKGROUND OF THE INVENTION

[0003] In 1992, the United States Environmental Protection Agencyadopted a standard for lead and copper in municipal drinking water. Thestandard sets action levels for lead and copper in standing samplescollected from faucets with the highest risk for elevated lead andcopper levels. The action level for lead is 15 μg/liter of water; theaction level for copper is 1.3 mg/liter of water. The source of theseand other metals in drinking water is primarily corrosion of plumbingsystem components, which include copper and lead-based solder as well ascarbon steel and brass. Current anticorrosion additives, which includephosphate salts and/or zinc salts, have been in use for many years, butdo not always provide adequate protection. There remains a need forimproved corrosion inhibitors which are not themselves likely to presenta health hazard when added to water.

[0004] Co-owned U.S. Pat. No. 5,202,028 and No. 5,510,057 describe theuse of stannous salts, such as stannous octanoate or stannous chloride,typically in combination with other additives, in an alcoholic solventmedium, for reducing metal corrosion in, for example, cooling watertower systems. Additional solubilizing agents were also generally used.However, these additional components, including the alcoholic solvent,would be undesirable or even prohibited for use in drinking watersupplies. Alcoholic solvents can provide a food source for the growth ofbacteria and thus impact the sterility of drinking water supplies.Reduction in microbial growth would be beneficial in industrial watersupplies as well. Accordingly, there is a need for effective corrosioninhibitors which minimize organic content.

SUMMARY OF THE INVENTION

[0005] The present invention includes, in one aspect, a method forinhibiting corrosion of corrodible metal in contact with water in adistributive water system. According to the method, a compositionconsisting essentially of a stannous (Sn⁺²) salt of a non-carbon acid,or an aqueous solution thereof, is added to the water, and aconcentration of the stannous salt corresponding to a tin level of about0.01 to about 75 ppm, preferably about 0.05 to 25 ppm, is maintained inthe water system.

[0006] The salt is preferably selected from stannous bromide, chloride,sulfate, nitrate, oxide, pyrophosphate, perchlorate, tetrafluoroborate,monofluorophosphate, ammonium fluoride, sodium fluoride, andfluorosilicate. Preferred salts include stannous bromide, chloride,sulfate, nitrate, and oxide. The stannous salt is preferably added inthe form of an aqueous solution.

[0007] The corrodible metal is typically a ferrous metal, a brass metal,a copper-containing metal, or a lead-containing metal. In a preferredembodiment of the method, the distributive water system is a municipaldrinking water system. Such a system may initially contain an alkalimetal phosphate, such as a pyrophosphate, an orthophosphate, ahexametaphosphate, a hypophosphate, a polyphosphate, or a combinationthereof, typically in an amount effective to provide a concentration ofabout 0.01 ppm to about 5 ppm of the alkali metal phosphate in the watersystem. The water system may also be treated to contain at least onecomponent selected from a dispersing agent, a chelating agent, and abiocide.

[0008] In another aspect, the invention provides a related method forinhibiting corrosion of corrodible metal in contact with water in adistributive water system. In this method, a composition consistingessentially of a stannous (Sn⁺²) salt of a non-carbon acid, or anaqueous solution thereof, is added to the water system, in combinationwith an alkali metal phosphate selected from a pyrophosphate, anorthophosphate, a hexametaphosphate, a hypophosphate, and apolyphosphate. A concentration of the stannous halide corresponding to atin level of about 0.01 to about 75 ppm is maintained in the watersystem. Again, the salt is preferably selected from stannous bromide,chloride, sulfate, nitrate, oxide, pyrophosphate, perchlorate,tetrafluoroborate, monofluorophosphate, ammonium fluoride, sodiumfluoride, and fluorosilicate, and more preferably selected from stannousbromide, chloride, sulfate, nitrate, and oxide. The alkali metalphosphate, e.g. sodium hexametaphosphate or sodium orthophosphate, istypically present in an amount effective to provide a concentration ofabout 0.01 ppm to about 5 ppm in the water system.

[0009] In a further aspect, the invention provides a metal corrosioninhibiting composition, consisting essentially of (i) a stannous salt ofa non-carbon acid, or an aqueous solution thereof, and (ii) an alkalimetal phosphate selected from a pyrophosphate, an orthophosphate, ahexametaphosphate, a hypophosphate, and a polyphosphate. In preferredembodiments, the stannous salt is selected from stannous bromide,chloride, sulfate, nitrate, oxide, pyrophosphate, perchlorate,tetrafluoroborate, monofluorophosphate, ammonium fluoride, sodiumfluoride, and fluorosilicate, and more preferably from stannous bromide,chloride, sulfate, nitrate, and oxide. The stannous salt and alkalimetal phosphate are present in relative amounts effective to produceconcentrations of about 0.01 to about 75 ppm tin and about 0.01 ppm toabout 5 ppm alkali metal phosphate, respectively, in a distributivewater system.

[0010] The composition may also include one or more substances typicallyadded to a distributive water stream for purposes other than inhibitionof corrosion, such as a dispersing agent, chelating agent, or biocide.In one embodiment, the composition includes an acrylate copolymer, e.g.an acrylate/sulfonate copolymer.

[0011] These and other objects and features of the invention will becomemore fully apparent when the following detailed description of theinvention is read in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION

[0012] I. Definitions

[0013] “Corrosion” of a metal in contact with water, as used herein,refers to degradation of the metal due to chemical reaction with itsenvironment, in this case, water and substances present in the water,including air. Such corrosion ultimately leads to dissolution ordispersion of the metal or metal compound in the water, observed as aloss of mass of the metal.

[0014] A “non-carbon” acid is an acid containing no carbon atoms, e.g.hydrochloric, sulfuric, phosphoric, or nitric acid.

[0015] As used herein, an “aqueous solution” of a stannous salt of anon-carbon acid refers to a composition of the salt in water, preferablyin fairly high concentration, i.e., about 30 weight percent or greater,if the salt is of sufficient solubility in water to achieve thisconcentration. The composition may include an acid, e.g. HCl, H₂SO₄, orHNO₃, to promote dissolution of the stannous salt. There are no othercomponents in any appreciable concentration.

[0016] A “composition consisting essentially of” a stannous salt of anon-carbon acid (or aqueous solution) refers to the stannous salt, oraqueous solution, in combination with one or more optional componentswhich do not materially affect the metal corrosion inhibitingcharacteristic(s) of the composition. Such components would typically besubstances added to a distributive water stream for purposes other thaninhibition of corrosion, as defined above. For example, dispersing orchelating agents, such as soluble anionic polymers (e.g. polyacrylatesor acrylate copolymers), may be employed to reduce scale formation onsolid surfaces; biocides may be added to reduce microbial growth.

[0017] A similar definition applies to compositions “consistingessentially of” a stannous salt of a non-carbon acid (or aqueoussolution) and an alkali metal phosphate. Such compositions may alsoinclude components present for purposes other than inhibition ofcorrosion.

[0018] A “concentration of stannous salt which corresponds to aconcentration of tin” in a given concentration range is determined fromthe relative molecular weights of the components, for example: 0.010 ppmtin corresponds to approx. 0.018 ppm SnSO₄, 0.016 ppm SnCl₂, or 0.023ppm SnBr₂; 0.150 ″ 0.27 ″ 0.24 ″ 0.35 ″ 1.0 ″ 1.81 ″ 1.60 ″ 2.34 ″ 10 ″18.1 ″ 16.0 ″ 23.4 ″ 75 ″ 136 ″ 120 ″ 176 ″

[0019] In the appended claims, water contained in a distributive watersystem to which a corrosion inhibitor is added is referred to as the“water system”, rather than simply “water”, to avoid confusion with theconcentrated aqueous stannous halide solutions.

[0020] II. Preparation of Aqueous Stannous Salt Solutions and Use inCorrosion Inhibition

[0021] To form the minimal organic content corrosion inhibitors of theinvention, any stannous salt of a non-carbon acid having sufficientaqueous solubility to be maintained in a water system at a concentrationcorresponding to a selected tin level in the range of about 0.01 toabout 75 ppm may be used. As noted above, preferred salts includestannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate,perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride,sodium fluoride, and fluorosilicate, and particularly preferred saltsinclude stannous bromide, chloride, sulfate, nitrate, and oxide.

[0022] In some cases, stable aqueous solutions are more readily preparedby adding water to the salt, rather than the reverse order of addition.For example, it was determined in the course of work in support of theinvention that highly concentrated aqueous stannous chloride solutions,i.e. up to 90 weight percent SnCl₂, can be conveniently prepared byadding water to stannous chloride in the desired weight ratio andstirring for a brief period of time. Dissolution was enhanced by usingslighly acidified water.

[0023] The salt or aqueous solution is added to a distributive watersystem in an amount needed to produce a desired concentration of tin, asdescribed above, in the water system. The concentration of Sn⁺² in thewater system may then be monitored, by techniques known in the art, andadjusted as necessary to maintain the desired concentration.

[0024] Aqueous stannous chloride solutions containing 50 weight percentSnCl₂ were prepared by adding water to the solid, in the mannerdescribed above. In the following experiments, samples of thesesolutions were added to samples of municipal drinking water effective toprovide final concentrations of 0.15 to 1.0 ppm stannous chloride in thewater, as shown.

[0025] To produce the data shown in Table 1, samples of the metals shownwere immersed in samples of municipal drinking water, containing about 3ppm of the conventional corrosion inhibitor sodium hexametaphosphate(HMP), from a system located in the upper midwestern region of the U.S.The test was carried out according to the procedure described inMaterials and Methods, below. Corrosion rates, expressed in mils peryear, were determined after 7 days at 120° F. in aerated water sampleswith and without 0.25 ppm added SnCl₂. As shown in the Table,significant decreases in corrosion were observed in the SnCl₂-treatedsamples, especially with respect to carbon steel, copper and leadcorrosion. TABLE 1 No added SnCl₂ 0.25 ppm SnCl₂ Metal Rate, MPY Rate,MPY Carbon Steel 40.2 2.1 Brass (CDA-443) 0.43 0.15 Copper 0.71 0.00Lead 1.80 0.09

[0026] To obtain the data shown in Table 2, a specimen of each metal wassuspended in a sample of municipal drinking water from Oklahoma(designated OKC) in a one-liter flask equipped with a condenser, gasdelivery inlet, and thermometer. The contents of each flask were heatedto 120° F. under a continuous air flow (1.5 cu ft/min) for seven days.Corrosion rates are again expressed as mils per year; nd=not determined.Various concentrations of SnCl₂ were used, as shown. As the data show,very low concentrations of SnCl₂ were effective in reducing corrosion.TABLE 2 Treated (SnCl₂) Rates, MPY No added SnCl₂* 0.15 0.25 0.50 1.0Metal Rate, MPY ppm ppm ppm ppm Lead 1.2 nd 0.13 0.06 0.00 CDA-443 Brass0.43 0.23 nd 0.02 nd CDA-120 Copper 0.71 nd 0.00 nd nd

[0027] II. Corrosion Inhibition by Stannous Chloride in the Presence ofPhosphate Additives

[0028] Currently, municipal water supplies frequently contain, ascorrosion inhibiting additives, phosphates and/or zinc salts. Forexample, samples of OKC drinking water used in the experiments describedherein contain, unless otherwise indicated, 3 ppm sodiumhexametaphosphate. Therefore, experiments were carried out to examinethe comparative and additive effects of aqueous stannous chloride andthe phosphate additives already in use.

[0029] Table 3 shows corrosion rates in MPY for various metals in OKCwater (aerated at 120° F., as described for Table 2) containing 3 ppmhexametaphosphate (as received) with and without other additives. Theresults show that increasing the amount of phosphate had variableresults, depending on the metal tested; in some cases corrosionincreased (e.g. for lead). Addition of zinc chloride gave moderateimprovements, except in the case of lead. Addition of stannous chloride,however, reduced corrosion significantly, as is most apparent from acomparison of the 3^(rd) and 5^(th) data columns. TABLE 3 Additive^(a) 3ppm HMP 3 ppm HMP 3 ppm HMP 3 ppm OP 6 ppm HMP 6 ppm HMP Metal (asreceived) 3 ppm OP 6 ppm HMP 0.5 ppm SnCl₂ 0.5 ppm SnCl₂ 0.1 ppm Zn⁺²1018 41.3 25.7 29.5 18.6 11.4 24.3 Carbon Steel CDA-443 0.49 0.28 1.400.19 0.02 0.98 Brass CDA-120 0.72 0.28 0.90 0.00 0.00 0.28 Copper Lead1.21 2.19 1.10 0.90 0.00 1.13

[0030] Table 4 gives further data for carbon steel corrosion (aerated at120° F., as described above), showing combinations of SnCl₂ with varyingamounts of phosphate. As shown in the Table, addition of SnCl₂ reducedcorrosion in all cases, although it appeared to be more effective atlower levels (3 ppm vs. 6 ppm) of phosphate. TABLE 4 (Carbon Steel) ppmHMP ppm SnCl₂ Corrosion, MPY 3 (as received) 0 41.3 3 0.25 6.8 3 0.507.6 6 (3 ppm added) 0 29.5 6 0.25 13.3 6 0.50 11.4 3 (as received, plus0.25 11.1 3 ppm OP added)

[0031] To produce the data in Table 5, a sample of OKC water wasobtained which had been filtered and disinfected but not treated withphosphate. Corrosion rates were determined for carbon steel in heated,aerated water as described above. TABLE 5 (Carbon Steel) ppm HMP ppmSnCl₂ Corrosion, MPY 0 0 49.3 3 0 21.1 3 0.25 13.3 0 0.25 2.1

[0032] III. Copper Corrosion Inhibition by SnCl₂ in the Presence of aPolymeric Dispersant or Metal Chelator

[0033] As noted above, the present anticorrosive stannous saltcompositions may be used in combination with components which do notmaterially affect the metal corrosion inhibiting characteristic(s) ofthe composition. Such components would typically be substances added toa distributive water stream for purposes other than inhibition ofcorrosion. For example, dispersing or chelating agents, such as solubleanionic polymers (e.g. polyacrylates or acrylate copolymers), may beemployed to reduce scale formation on solid surfaces. One such polymer,Acumer® 2100 (Rohm & Haas), an acrylate/sulfonate copolymer, was addedto test systems such as described above. In these tests, the metalcoupons were copper (CDA-110), and the water was municipal drinkingwater from LaSalle, Ill., which contained no phosphate additives. Testconditions were as described in Materials and Methods, below (120° F.,aerated water, 7 days duration).

[0034] As shown below, the copolymer had no adverse properties on thecorrosion inhibiting properties of SnCl₂. Although larger concentrationsof the copolymer alone appeared to increase corrosion, this effect wasvery effectively counteracted by the presence of SnCl₂.

[0035] Also shown in Table 6 is a combination of SnCl₂ with AMP(aminotris(methylenephosphonic acid); Mayoquest® 1320), which is usefulin scale control by sequestering calcium and magnesium as well asferrous ion. Again, the metal coupon showed very low corrosion in thepresence of this additive and low concentrations of SnCl₂. TABLE 6(Copper) Corrosion Additive SnCl₂, ppm Rate, MPY Acumer ® 2100, 0.25 ppm0 1.80 Acumer ® 2100, 0.25 ppm 0.25 0.0 Acumer ® 2100, 3 ppm 0 29.3Acumer ® 2100, 3 ppm 0.50 0.0 AMP, 0.25 ppm 0 1.80 AMP, 0.25 ppm 0.250.08

[0036] IV. Lead Corrosion Inhibition at Varying pH

[0037] A series of tests was run to determine the effectiveness ofconventional corrosion inhibitors and SnCl₂ on lead coupons at varyingpH. Drinking water from Fall River, Mass., having no phosphate additivesas received, was used for the tests, using the methods described above.pH was determined using EM Reagents ColorpHast® indicator strips. TABLE7 (Lead) Corrosion pH Treatment Rate, MPY 5.3 None 0.64 5.3^((a)) 3 ppmOP 3.43 5.3 3 ppm HMP 4.60 7.2 None 2.00 7.2 3 ppm OP 3.39 7.2 3 ppm HMP2.10 7.2 0.25 ppm SnCl₂ 0.03

[0038] As shown in Table 7, corrosion was greater in untreated water athigher pH, which may be partially due to the somewhat higher solubilityof lead oxide under these conditions. Both sodium orthophosphate (OP)and sodium hexametaphosphate (HMP) increased the corrosion, particularlyunder acidic conditions, producing a reddish-yellow corrosion product(lead oxides) on the metal. However, corrosion was minimal in the samplecontaining 0.25 ppm SnCl₂.

[0039] While the invention has been described with reference to specificmethods and embodiments, it will be appreciated that variousmodifications may be made without departing from the invention.

MATERIALS & METHODS

[0040] A typical corrosion test was carried out in the following manner.The test sample of municipal drinking water was placed in a one literresin kettle equipped to hold (1) a jacketed water cooled condenser, (2)a mounted air-injection gas tube, and (3) a holder from which the testspecimen was suspended and totally immersed in the test water. Thecomplete assembly was placed in a constant temperature oil bath. Thetest water was agitated by compressed air at a flow rate of 1.5 ft³/min.The temperature was maintained at 120° F. for seven days.

[0041] Test coupons were generally 1″×2″×{fraction (1/16)}″ in size,with a ¼″ mounting hold centered ¼″ from one end. The test coupons wereweighed before and after testing, and weight loss in milligrams wasconverted to a corrosion rate of milliinches per year (mpy).

It is claimed:
 1. A method for inhibiting corrosion of corrodible metalin contact with water in a distributive water system, comprising (a)adding to said water system, a composition consisting essentially of astannous salt of a non-carbon acid, or an aqueous solution thereof, and(b) maintaining in said water system a concentration of said stannoussalt which corresponds to a concentration of tin between about 0.01 ppmand about 75 ppm.
 2. The method of claim 1 , wherein the stannous saltis selected from the group consisting of stannous bromide, chloride,sulfate, nitrate, oxide, pyrophosphate, perchlorate, tetrafluoroborate,monofluorophosphate, ammonium fluoride, sodium fluoride, andfluorosilicate.
 3. The method of claim 2 , wherein the stannous salt isselected from the group consisting of stannous bromide, chloride,sulfate, nitrate, and oxide.
 4. The method of claim 1 , wherein theconcentration of stannous salt is maintained in the water system at aconcentration which corresponds to a concentration of tin between about0.05 ppm and about 25 ppm.
 5. The method of claim 1 , wherein in step(i) the stannous salt is added in the form of an aqueous solution. 6.The method of claim 1 , wherein the corrodible metal is selected fromthe group consisting of a ferrous metal, a brass metal, acopper-containing metal, and a lead-containing metal.
 7. The method ofclaim 1 , wherein the distributive water system is a municipal drinkingwater system.
 8. The method of claim 1 , wherein the water systemcontains an alkali metal phosphate selected from a pyrophosphate, anorthophosphate, a hexametaphosphate, a hypophosphate, and apolyphosphate.
 9. The method of claim 8 , wherein the alkali metalphosphate is present in an amount effective to provide a concentrationof about 0.01 ppm to about 5 ppm in the water system.
 10. The method ofclaim 1 , wherein the water system is treated to contain at least onecomponent selected from a dispersing agent, a chelating agent, and abiocide.
 11. A method for inhibiting corrosion of corrodible metal incontact with water in a distributive water system, comprising (a) addingto said water system, a composition consisting essentially of (i) astannous salt of a non-carbon acid, or an aqueous solution thereof, abd(ii) an alkali metal phosphate selected from a pyrophosphate, anorthophosphate, a hexametaphosphate, a hypophosphate, and apolyphosphate, and (b) maintaining in said water system a concentrationof said stannous salt which corresponds to a concentration of tinbetween about 0.01 ppm and about 75 ppm.
 12. The method of claim 11 ,wherein the stannous salt is selected from the group consisting oftannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate,perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride,sodium fluoride, and fluorosilicate.
 13. The method of claim 12 ,wherein the stannous salt is selected from the group consisting ofstannous bromide, chloride, sulfate, nitrate, and oxide.
 14. The methodof claim 11 , wherein the concentration of stannous salt is maintainedin the water system at a concentration which corresponds to aconcentration of tin between about 0.05 ppm and about 25 ppm.
 15. Themethod of claim 1 , wherein in step (i) the stannous salt is added inthe form of an aqueous solution.
 16. The method of claim 11 , whereinthe alkali metal phosphate is added in an amount effective to provide aconcentration of about 0.01 ppm to about 5 ppm in said water system. 17.The method of claim 11 , wherein the alkali metal phosphate is sodiumhexametaphosphate or sodium orthophosphate.
 18. A metal corrosioninhibiting composition for use in a distributive water system,consisting essentially of (i) a stannous salt of a non-carbon acid or anaqueous solution thereof, and (ii) an alkali metal phosphate selectedfrom a pyrophosphate, an orthophosphate, a hexametaphosphate, ahypophosphate, and a polyphosphate.
 19. The composition of claim 18 ,wherein the stannous salt is selected from the group consisting ofstannous bromide, chloride, sulfate, nitrate, oxide, pyrophosphate,perchlorate, tetrafluoroborate, monofluorophosphate, ammonium fluoride,sodium fluoride, and fluorosilicate.
 20. The composition of claim 19 ,wherein the stannous salt is selected from the group consisting ofstannous bromide, chloride, sulfate, nitrate, and oxide.
 21. Thecomposition of claim 20 , wherein the stannous salt is stannous sulfate.22. The composition of claim 18 , wherein the stannous salt and thealkali metal phosphate are present in relative amounts effective toproduce concentrations of about 0.01 to about 75 ppm tin and about 0.01ppm to about 5 ppm alkali metal phosphate, respectively, in adistributive water system.
 23. The composition of claim 18 , furthercomprising at least one component selected from a dispersing agent, achelating agent, and a biocide.
 24. The composition of claim 23 ,wherein said component is an acrylate copolymer.